Optical sheet, screen, and display apparatus

ABSTRACT

An optical sheet, a screen, and a display apparatus capable of reducing speckles are provided. An optical sheet (50) includes a particle layer (55) including a transparent retaining part (56) having a predetermined thickness and a particle (60) that is accommodated in a cavity (56a) formed in the retaining part (56) and includes a first portion (61) and a second portion (62) having different dielectric constants. The first portion (61) includes a transparent first main portion (66a) and a first diffusion component (66b) that diffuses light. The second portion (62) includes a transparent second main portion (67a) and a second diffusion component (67b) that diffuses light. The first diffusion component (66b) and the second diffusion component (67b) have a diameter d satisfying the following conditional expression (1): 0.1 μm≤d≤15 μm (1).

TECHNICAL FIELD

The present invention relates to an optical sheet, an image-displayingscreen using the optical sheet, and a display apparatus including thescreen.

BACKGROUND ART

For example, as disclosed in Patent Literature 1 and Patent Literature2, projectors using a coherent light source are widely used. Forcoherent light, laser light oscillated from a laser light source istypically used. If image light from a projector is formed by coherentlight, speckles are observed on a screen that is irradiated with theimage light. Speckles are perceived as a dot pattern and deterioratedisplay image quality. In Patent Literature 1, the incident angle of theimage light incident on each position on the screen is temporallychanged for the sake of reducing speckles. As a result, uncorrelatedscattering patterns occur on the screen, and speckles can be reduced bythe superposition of the scattering patterns.

CITATION LIST Patent Literature

Patent Literature 1: International Publication No. 2012/033174

Patent Literature 2: JP 2008-310260 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As another method for reducing speckles, it maybe effective totemporally change the diffusion characteristic of the screen. PatentLiterature 2 proposes a screen that is made of electronic paper. Thescreen according to Patent Literature 2 changes its reflectanceaccording to an irradiation position of image light projected by araster scan method.

If the reflectance of areas not irradiated with the image light iscontrolled to be low, the reflection of ambient light, such as externallight and illumination light, from the areas of low reflectance can besuppressed to display a high-contrast image.

However, according to the screen disclosed in Patent Literature 2, thereflectance is only changed by a display ratio of white particles andblack particles, with no effect on speckles occurring on the screen. Toeffectively reduce speckles occurring on a screen, it is effective totemporally change the diffusion wavefront while maintaining thediffusion characteristic of the screen. Methods such as directlyvibrating a screen have heretofore been proposed. With many restrictionsin terms of practical use, such methods have not yet become widelyprevalent.

The present invention has been achieved in view of the foregoing, and itis an object thereof to provide an optical sheet, a screen, and adisplay apparatus which can sufficiently reduce speckles by a methoddifferent from the conventional ones.

Means for Solving the Problems

An optical sheet according to an embodiment of the present invention forachieving the foregoing object includes

a particle layer including

a transparent retaining part that has a predetermined thickness, and

a particle that is accommodated in a cavity formed in the retaining partand includes a first portion and a second portion having differentdielectric constants, wherein

the first portion includes a transparent first main portion and a firstdiffusion component that diffuses light,

the second portion includes a transparent second main portion and asecond diffusion component that diffuses light, and

the first diffusion component and the second diffusion component have adiameter d satisfying the following conditional expression (1):

0.1 μm<d<15 μm   (1)

In the optical sheet according to an embodiment of the presentinvention,

a volume fraction Vi indicating a proportion of a sum of volumes of thefirst diffusion component and the second diffusion component to a volumeof the particle satisfies the following conditional expression (2):

Vi≥3%   (2)

In the optical sheet according to an embodiment of the presentinvention,

a refractive index difference An between the first and second diffusioncomponents and the first and second main portions satisfies thefollowing conditional expression (3):

|Δn|<0.2   (3)

A screen according to an embodiment of the present invention includes:

the optical sheet; and

an electrode that forms an electric field for driving the particle ofthe particle layer when a voltage is applied thereto.

In the screen according to an embodiment of the present invention,

the electrode has a function of diffusing and reflecting light as adiffuse reflection layer.

A display apparatus according to an embodiment of the present inventionincludes:

the screen; and

a projector that irradiates the screen with coherent light.

The display apparatus according to an embodiment of the presentinvention further includes

a power source that applies a voltage to the electrode of the screen;and

a control device that controls a voltage applied from the power sourceto the electrode, wherein

the control device controls the applied voltage from the power source sothat the particle operates in the particle layer.

In the display apparatus according to an embodiment of the presentinvention,

the control device controls the applied voltage from the power source torepeatedly rotate the particle within an angular range of less than180°.

In the display apparatus according to an embodiment of the presentinvention,

the control device controls at least either one of a direction and aposition of the particle by the applied voltage from the power source sothat the first portion covers at least part of the second portion froman observer side along a normal direction of the screen.

Advantages of the Invention

With the optical sheet, the image-displaying screen using the opticalsheet, and the display apparatus including the screen according to anembodiment of the present invention, speckles can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a display apparatus according to the presentembodiment.

FIG. 2 illustrates a method for irradiating a screen of the displayapparatus according to the present embodiment with light.

FIG. 3 illustrates a part of a cross section of the screen of thedisplay apparatus according to the present embodiment.

FIG. 4 illustrates an operation of a particle in a particle layer of thescreen according to the present embodiment.

FIG. 5 illustrates a simulation model.

FIG. 6 illustrates a definition of scattered light in the simulation.

FIG. 7 illustrates results of scattering with respect to a volumefraction in the simulation.

FIG. 8 illustrates results of scattering with respect to a refractiveindex difference in the simulation.

FIG. 9 illustrates an example of a voltage applied to the screen.

FIG. 10 illustrates a part of a cross section of a screen according toanother embodiment.

FIG. 11 illustrates a particle in an example where a first portion and asecond portion have different colors.

FIG. 12 illustrates a particle in an example where the first portion andthe second portion have different volume ratios.

FIG. 13 illustrates an example of a screen including a plurality oflinear electrode portions.

FIG. 14 illustrates apart of a cross section of a transmission typescreen according to another embodiment.

FIG. 15 illustrates an example of a display apparatus using thetransmission type screen according to the present embodiment.

FIG. 16 illustrates another example of a Fresnel lens.

FIG. 17 illustrates a second example of a viewing angle expansion part.

FIG. 18 illustrates a third example of the viewing angle expansion part.

FIG. 19 illustrates another example of a diffusion part.

FIGS. 20A and 20B illustrate the concept of a cavity to a particle.

BEST MODE FOR CARRYING OUT THE INVENTION

A screen and a display apparatus according to the present invention willbe described below with reference to the drawings.

FIG. 1 illustrates a display apparatus 10 according to the presentembodiment.

The display apparatus 10 of transmission type according to the presentembodiment includes a projector 20, a screen 40 to be irradiated withimage light from the projector 20, a lenticular lens 70 arranged on theprojector 20 side of the screen 40, a Fresnel lens 80 arranged on theprojector 20 side of the lenticular lens 70, and non-illustrated blackstripes. The lenticular lens 70 may be arranged on a second surface 40 bside of the screen 40. A microlens array may be used instead of thelenticular lens 70. In such a case, a black matrix or a pinhole arraymay be used instead of the black stripes.

As will be described later, the screen 40 can temporally change itsdiffusion characteristic on incident light, whereby speckles can be madeless noticeable. Concerning such a function of the screen 40, thedisplay apparatus 10 includes a power source 30 and a control device 35.The power source 30 applies a voltage to the screen 40. The controldevice 35 adjusts the voltage applied from the power source 30 tocontrol the state of the screen 40. The control device 35 may control anoperation of the projector 20. For example, the control device 35 may bea general-purpose computer.

The projector 20 projects light for forming an image, i.e., image lighton the screen 40. In the illustrated example, the projector 20 includesa coherent light source 21 which oscillates coherent light, and anon-illustrated scanning device which adjusts an optical path of thecoherent light source 21. For example, the coherent light source 21includes a laser light source for oscillating laser light. The coherentlight source 21 may include a plurality of coherent light sources forgenerating light of different wavelength bands. In the case of thetransmission type screen 40, an observer E can observe an imagetransmitted through the screen 40 from the side of a second surface 40 bopposite from a first surface 40 a where the coherent light source 21is. The projector 20 may be configured to include a non-illustratedcontrol portion different from the control device 35 and be controlledby the internal control portion.

FIG. 2 illustrates a method for irradiating the screen of the displayapparatus according to the present embodiment with light.

In the illustrated example, the projector 20 projects coherent light onthe screen 40 by a raster scan method. As illustrated in FIG. 2, theprojector 20 projects the coherent light to scan the entire area on thescreen 40. The scanning is performed at high speed. The projector 20stops the emission of the coherent light from the coherent light source21 according to an image to be formed. In other words, the coherentlight is projected only on positions to form the image on the screen 40.As a result, the image is formed on the screen 40. The operation of theprojector 20 is controlled by the control device 35.

FIG. 3 illustrates a part of a cross section of the screen of thedisplay apparatus according to the present embodiment.

The screen 40 will initially be described. In the example illustrated inFIG. 3, the screen 40 includes an optical sheet 50 including a pluralityof particles, and electrodes 41 and 42 connected to the power source 30.A first electrode 41 spreads flat over one main surface of the opticalsheet 50. A second electrode 42 spreads flat over the other main surfaceof the optical sheet 50. The screen 40 illustrated in FIG. 3 includes afirst cover layer 46 which covers the first electrode 41 to form oneoutermost surface of the screen 40, and a second cover layer 47 whichcovers the second electrode 42 to form the other outermost surface ofthe screen 40.

The first electrode 41, the first cover layer 46, the second electrode42, and the second cover layer 47 for the image light to be transmittedthrough are preferably transparent and each have a transmittance of 80%or higher, more preferably 84% or higher, in the visible light regioneach. The visible light transmittance is determined as an average valueof transmittances at respective wavelengths, measured in the range ofmeasurement wavelengths of 380 nm to 780 nm by using a spectrophotometer(“UV-3100PC” made by Shimadzu Corporation, a product compliant with JISK0115).

ITO (Indium Tin Oxide), InZnO (Indium Zinc Oxide), Ag nanowires, carbonnanotubes, and the like maybe used as a conductive material for formingthe first electrode 41. The first cover layer 46 is a layer forprotecting the first electrode 41 and the optical sheet 50. The firstcover layer 46 may be made of a transparent resin, such as polyethyleneterephthalate, which has excellent stability, polycarbonate, acrylicresin, methacrylic resin, or cycloolefin polymer. The second electrode42 may be configured similarly to the first electrode 41. The secondcover layer 47 may be configured similarly to the first cover layer 46.

The optical sheet 50 includes a pair of substrates 51 and 52, and aparticle layer 55 arranged between the pair of substrates 51 and 52. Thefirst substrate 51 supports the first electrode 41, and the secondsubstrate 52 supports the second electrode 42. The particle layer 55 issealed in between the first substrate 51 and the second substrate 52.

The first substrate 51 and the second substrate 52 are made of amaterial having a strength capable of sealing the particle layer 55 andfunctioning as a support for the first electrode 41, the secondelectrode 42, and the particle layer 55. For example, the firstsubstrate 51 and the second substrate 52 are made of a polyethyleneterephthalate resin film or the like. In the example illustrated in FIG.3, the image light is transmitted through the first substrate 51 of thescreen 40. The first substrate 51 therefore preferably is transparentand has a visible light transmittance similar to that of the firstelectrode 41 and the first cover layer 46. In particular, the secondsubstrate 52 also preferably has a visible light transmittance similarto that of the first electrode 41 and the first cover layer 46.

The particle layer 55 includes a large number of particles 60 and aretaining part 56 for retaining the particles 60. The retaining part 56retains the particles 60 in an operable manner. As illustrated in FIG.3, the retaining part 56 includes a large number of cavities 56 a, andthe particles 60 are accommodated in the respective cavities 56 a. Eachcavity 56 a has inner dimensions greater than outer dimensions of theparticles 60 in the cavity 56 a. The particles 60 can thus be operablein the cavities 56 a. The retaining part 56 is swollen with liquid 57.In the cavities 56 a, the gaps between the retaining part 56 and theparticles 60 are filled with the liquid 57. The retaining part 56swollen with the liquid 57 can stably ensure a smooth operation of theparticles 60.

The liquid 57 is used to smoothen the operation of the particles 60. Theliquid 57 is retained in the cavities 56 a by the swelling of theretaining part 56 with the liquid 57. The liquid 57 preferably has lowpolarity so as not to hinder the particles 60 from operating in responseto an electric field. Various materials for smoothing the operation ofthe particles 60 may be used as the low polarity liquid 57. For example,dimethyl silicone oil, isoparaffin liquids, straight-chain alkanes, andthe like may be used as the liquid 57.

For example, an elastomer sheet and the like made of an elastomermaterial may be used as the retaining part 56. The retaining part 56made of an elastomer sheet can be swollen with the liquid 57. Forexample, silicone resin, (slightly crosslinked) acrylic resin, (slightlycrosslinked) styrene resin, polyolefin resin, and the like may be usedas the material of the elastomer sheet.

The particles 60 have a function of changing the traveling direction ofthe image light projected from the projector 20 illustrated in FIG. 1,for example, a function of diffusing, reflecting, or refracting theimage light. The particles 60 include a first portion 61 and a secondportion 62 having different dielectric constants. If the particles 60are placed in an electric field, electric dipole moment thus occurs inthe particles 60. Here, the particles 60 operate toward a position wherethe vector of the dipole moment is reverse to the vector of the electricfield.

If a voltage is applied between the first electrode 41 and the secondelectrode 42 and an electric field occurs in the optical sheet 50 lyingbetween the first electrode 41 and the second electrode 42, theparticles 60 operate in the cavities 56 a into an orientation stablewith respect to the electric field, i.e., a position and directionstable with respect to the electric field. The screen 40 changes itsdiffusion wavefront according to the operation of the particles 60having a light diffusion function.

For example, the control device 35 can repeatedly rotate the particles60 within an angular range of less than 180° by controlling the appliedvoltage from the power source 30. At least either one of the firstportion 61 and the second portion 62 can thus be selectively positionedon the observer side.

The control device 35 can also control at least either one of thedirection and position of each particle 60 by using the applied voltagefrom the power source 30 so that the first portion 61 of the particle 60covers at least part of the second portion 62 from the observer sidealong a normal direction of the screen 40. Consequently, even if thefirst portion 61 and the second portion 62 do not have exactly the samecolor, a change in the color tone of the screen 40 can be effectivelymade less perceptible while an image is displayed with the particles 60in operation.

The particles 60 including the first portion 61 and the second portion62 having different dielectric constants can be manufactured by variousmethods including known techniques. For example, a method for arrangingorganic or inorganic spherical particles in a single layer by using anadhesive tape or the like, and evaporating a layer of positively andnegatively charged resin components or an inorganic layer different fromthe spherical particles on the hemispherical surfaces (evaporationmethod; for example, see JP 56-67887 A), a method using a rotating disk(for example, see JP 6-226875 A), a method for bringing two types ofliquid droplets having different dielectric constants into contact inthe air by using a spray method or inkjet method (for example, see JP2003-140204 A), a microchannel method (for example, see JP 2004-197083A), and the like are used.

As proposed in JP 2004-197083 A, the first portion 61 and the secondportion 62 having different dielectric constants can be formed by usingmaterials having different charging characteristics. In general, themicrochannel method uses a continuous phase and a particle forming phasewhich have an oil-based/water-based (O/W type) or water-based/oil-based(W/O type) relationship with each other. A continuous phase containingtwo types of materials having different charging characteristics issequentially discharged from a first microchannel for transporting thecontinuous phase into a particle forming phase of a moving mediumflowing in a second microchannel, whereby two-phase polymer particles60, or dipolar particles 60, having (±) charge polarities aremanufactured.

In a microchannel method according to the present embodiment, acontinuous phase is initially formed by separating, in an oil- orwater-based moving medium containing polymeric resin components,polymeric resin components insoluble to the medium. The polymeric resincomponents in the continuous phase are made of mutually-differentpositively and negatively charged polymeric monomers. Next, thepolymeric monomers are transported to a first microchannel, and then thecontinuous phase is sequentially discharged, either continuously orintermittently, into a water- or oil-based particle forming phaseflowing through a second microchannel. Since the article discharged intothe particle forming phase forms particles in the course of discharge,distribution, and transportation in the microchannel, the polymericresin components in the particles are then polymerized and cured by UVirradiation and/or heating. In such a manner, the particles 60 areprepared as appropriate.

Among the polymeric resin components used for the particles 60 aremonomer species that have a tendency to exhibit (−) chargeability and(+) chargeability, respectively, depending on the type of functionalgroup or substituent. If at least two or more, a plurality of types ofmonomers is used as the polymeric resin components, a plurality ofmonomers, preferably ones having a tendency toward the same type ofchargeability, are then suitably used in combination as appropriate bymaking the tendencies to exhibit (+) chargeability and (−) chargeabilitypublicly known. Additives other than the monomers, like a polymerizationinitiator, are prepared and added so that the entire material will notlose chargeability.

If the polymeric resin components contain at least one type offunctional group and/or substituent, examples of the functional group orsubstituent may include a carbonyl group, a vinyl group, a phenyl group,an amino group, an amide group, an imide group, a hydroxyl group, ahalogen group, a sulfonate group, an epoxy group, and a urethane bond.Monomer species including such a functional group or substituent ofpolymeric monomer may be suitably used singly or in combination of twotypes or more as appropriate. Polymeric monomers proposed in JP2004-197083 may be used as the ones having a tendency to exhibit (−)chargeability or (+) chargeability.

In manufacturing the particles 60 by the microchannel method, the outershape of the resulting particles 60, the shape of the interface betweenthe first portion 61 and the second portion 62 in each particle 60, andthe like can be adjusted by adjusting the speeds, merging directions,and the like of the two types of polymeric resin components constitutingthe continuous phase during merging, and the speed, discharge direction,and the like of the continuous phase during discharge into the particleforming phase.

In the examples of the particles 60 illustrated in FIG. 3, the firstportion 61 and the second portion 62 have the same volume ratios. Theinterface between the first portion 61 and the second portion 62 in theparticles 60 is formed in a flat shape. In other words, the particles 60are formed in a spherical shape, and the first portion 61 and the secondportion 62 are formed in a hemispherical shape.

FIG. 4 illustrates a particle 60 in the particle layer 55 of the screen40 according to the present embodiment.

If the two types of polymeric resin components constituting thecontinuous phase include diffusion components, an internal diffusionfunction can be given to the first portion 61 and the second portion 62of the particle 60. As illustrated in FIG. 4, the first portion 61 ofthe particle 60 includes a first main portion 66 a and first diffusioncomponents 66 b distributed in the first main portion 66 a. Similarly,the second portion 62 of the particle 60 includes a second main portion67 a and second diffusion components 67 b distributed in the second mainportion 67 a.

That is, the spherical particle 60 illustrated in FIG. 4 can exert adiffusion function on light traveling through the first portion 61 andlight traveling through the second portion 62. Here, the first diffusioncomponents 66 b and the second diffusion components 67 b refer tocomponents that can act on light traveling through the particle 60 tochange the traveling direction of the light by reflection, refraction,etc. For example, such a light diffusion function of the first diffusioncomponents 66 b and the second diffusion components 67 b is provided bymaking the first diffusion components 66 b and the second diffusioncomponents 67 b of a material having a refractive index different fromthat of the material constituting the first main portion 66 a and thesecond main portion 67 a of the particle 60, or by making the firstdiffusion components 66 b and the second diffusion components 67 b of amaterial that can cause reflection of the light.

Examples of the first diffusion components 66 b and the second diffusioncomponents 67 b having a refractive index different from that of thematerial constituting the first main portion 66 a and the second mainportion 67 a include resin beads, glass beads, metal compounds, andgas-containing porous substances. The first diffusion components 66 band the second diffusion components 67 b may simply be air bubbles.

The particles 60 preferably have a single color. In other words, thefirst portion 61 and the second portion 62 preferably have the samecolor. The colors of the first portion 61 and the second portion 62 canbe adjusted by adding coloring materials such as a pigment and dye.Pigments and dyes disclosed in JP 2005-99158 A, Japanese Patent No.2780723, Japanese Patent No. 5463911, and the like may be used.

The single color used for the particles 60 means that even if theparticles 60 operate in the optical sheet 50 without an image displayedon the screen 40, the particles 60 have uniform color such that theobserver observing the screen 40 illustrated in FIG. 1 with normalobservation power is not able to perceive a change in the color of thescreen 40. In other words, if the second surface 40 b of the screen 40in a state where the first portions 61 of the particles 60 face thefirst surface 40 a of the screen 40 and the second surface 40 b of thescreen 40 in a state where the second portions 62 of the particles 60face the first surface 40 a of the screen 40, without an imagedisplayed, are observed by the observer with normal observation powerand perceived to have the same color, the particles 60 have a singlecolor.

Specifically, the second surface 40 b of the screen 40 in the statewhere the first portions 61 of the particles 60 face the first surface40 a of the screen 40 and the second surface 40 b of the screen 40 inthe state where the second portions 62 of the particles 60 face thefirst surface 40 a of the screen 40 preferably have a color differenceΔE*ab (=[(ΔL*)²+(Δa*)²+(Δb*)²]^(1/2)) of 1.5 or less. The colordifference ΔE*ab is a value determined based on lightness ΔL* andchromaticities a* and b* measured by using a colorimeter (CM-700d) madeby KONICA MINOLTA, INC., in conformity with JIS 28730. Evaluation ismade in terms of the value of the color difference ΔE*ab determinedbased on the lightness ΔL* and chromaticities a* and b* of transmittedlight.

Since the particles 60 have a single color, the color of the screen 40can be made constant when no image is displayed. When an image is to bedisplayed on the screen 40, a change in color tone is less likely to beperceived. As a result, deterioration in image quality due to a colorchange of the screen 40 can be effectively avoided.

For example, the particle layer 55, the optical sheet 50, and the screen40 are manufactured as described below.

For example, the particle layer 55 can be manufactured by a methoddisclosed at JP 1-28259 A. Ink is initially formed by distributingparticles 60 in polymeric silicone rubber. Next, the ink is spread overa flat substrate by a coater or the like, and polymerized into a sheetby heating, drying, etc. By such a procedure, the retaining part 56retaining the particles 60 is obtained. Next, the retaining part 56 isimmersed in liquid 57, such as a silicone oil, for a certain period. Theretaining part 56 is then swollen to form gaps filled with the liquid57, between the retaining part 56 made of silicon rubber or the like andthe particles 60. This forms cavities 56 a accommodating the liquid 57and the particles 60, whereby the particle layer 55 is manufactured.

Next, the screen 40 is manufactured by using the particle layer 55 by amanufacturing method disclosed in JP 2011-112792 A. The particle layer55 is initially covered with the pair of first and second substrates 51and 52 illustrated in FIG. 4. The particle layer 55 is sealed by usinglamination, an adhesive, or the like. The optical sheet 50 is therebymanufactured. Next, a first electrode 41 and a second electrode 42 arearranged on the optical sheet 50. A first cover layer 46 and a secondcover layer 47 are further stacked thereon, whereby the screen 40 isobtained.

A large-sized optical sheet 50 and screen 40 can be easily manufacturedby using such a method.

Next, a relationship between the first and second main portions 66 a and67 a and the first and second diffusion components 66 b and 67 b of theparticle 60 will be described.

The first diffusion components 66 b and the second diffusion components67 b preferably have a diameter d that satisfies the followingconditional expression (1):

0.1 μm<d<15 μm.   (1)

If the diameter of the first diffusion components 66 b and the seconddiffusion components 67 d is 0.1 μm or less, the scatteringcharacteristic varies greatly between R, G, and B colors of the laserprojector due to the effect of Rayleigh scattering. For example, thedegree of scattering increases in order of short-wavelength blue, green,and red. The screen 40 using such particles can cause color unevenness,for example, at locations such as the center and four corners, ordepending on the angle of viewing.

If the diameter of the first diffusion components 66 b and the seconddiffusion components 67 b is 15 μm or more, the large size of thediffusion components with respect to the particles 60 can causevariations in the volume ratio of the diffusion components particle byparticle, or hinder the particles 60 from maintaining the sphericalshape. If the diameter of the particles 60 is too large, aparticle-to-particle distance increases and can cause through lightwhich is light emitted from the projector and reflected or transmittedwithout impinging on any particle in the particle layer 55. The diameterof the particles 60 is thus preferably as small as possible. As thediameter d of the diffusion components increases, the curvature of theinterfaces between the main portions and the diffusion componentsdecreases. This reduces the angle at which the traveling direction ofthe light is curved by refraction, and is disadvantageous in terms ofthe speckle reduction effect. For the foregoing reasons, the diameter ofthe first diffusion components 66 b and the second diffusion components67 b is desirably 15 μm or less. If the diameter d of the diffusioncomponents is large, the light is diffused by the refraction at theinterfaces between the main portions and the diffusion components.According to a simulation to be described later, the calculation resultsof geometrical optics and Mie scattering are known to coincide if d issufficiently large.

A volume fraction Vi indicating the proportion of the sum of the volumesof the first diffusion components 66 b and the second diffusioncomponents 67 b to the volume of the particle 60 preferably satisfiesthe following conditional expression (2):

Vi≥3%   (2)

A refractive index difference Δn between the first and second diffusioncomponents 66 b and 67 b and the first and second main portions 66 a and67 a preferably satisfies the following conditional expression (3):

|Δn|≥0.2   (3)

Now, a simulation by which the conditional expression (2) of the volumefraction and the conditional expression (3) of the refractive indexdifference are determined will be described.

FIG. 5 illustrates a simulation model.

The simulation model includes a rectangular solid 101 simulating theretaining part 56 and a cavity 57 illustrated in FIG. 3, and a sphere102 simulating a particle 60 embedded in the rectangular solid 101. Therectangular solid 101 has a thickness of 144 μm. The refractive index ofthe rectangular solid 101 with respect to a wavelength of 550 nm is 1.4.The sphere 102 has a diameter of 90 μm and is assumed to be acrylicresin. The rectangular solid 101 is irradiated with a parallel beam of awavelength of 550 nm from a circular light source 100 having a diameterof 89.98 μm. Reflection or transmittance at each interface follows theFresnel reflection. The beam is not split, and either reflection ortransmission is selected in a stochastic manner. The cavity 57 is filledwith silicone oil, and the silicone oil and the silicone rubberconstituting the retaining part 56 have almost the same refractiveindexes. The cavity 57 is thus omitted in the model.

The particle number density, refractive index, optical density, orparticle size of the sphere 102 was set on the assumption that diffusionfollowed the Mie scattering. The particle number density (unit: /mm³)was calculated and set in advance from the volume fraction. Therefractive index was set to 1.43, the optical density 10000, and theparticle sizes of all particles 1000 nm. Since the convertedtransmittance is 10⁻¹⁰⁰⁰⁰, a beam incident on the sphere 102 isconsidered to be absorbed. That is, a beam received at infinity from thesystem here is one that is emitted from the rectangular solid 101without being incident on the sphere 102 at all. Such a beam does notcontribute to speckle reduction, and is desirably reduced as much aspossible.

FIG. 6 illustrates the definition of scattered light in the simulation.FIG. 7 illustrates a result of scattering with respect to the volumefraction in the simulation.

As illustrated in FIG. 6, with the direction of emission from the lightsource 100 to the sphere 102 as 0° and with the counterclockwisedirection as positive, the scattered light of −90°<θ<90° will bereferred to as forward scattering, and 90°<θ<270° as backwardscattering.

FIG. 7 illustrates the result of scattering of 1,000,000 incident beams(550 nm in wavelength) with respect to the volume fraction of the sphere102. As illustrated in FIG. 7, if the volume fraction is less than 3%, alot of beams are calculated to be received at infinity. Since such beamsdo not contribute to speckle reduction, the volume fraction Vipreferably satisfies the conditional expression (2) of being 3% or more.

FIG. 8 illustrates the results of scattering with respect to therefractive index difference in the simulation.

Next, calculations were performed by changing the refractive index, witha volume fraction of 3% and a transmittance of 100%. As illustrated inFIG. 8, if the refractive index difference |Δn| is smaller than 0.2,i.e., the conditional expression (3) is satisfied, the forwardscattering exceeds the backward scattering, and the transmitted beamsincrease. For example, if the rectangular solid 101 is made of siliconerubber, the sphere 102 may be made of an inorganic material such assilica, or an organic material such as polystyrene.

Next, an operation when an image is displayed by using the displayapparatus 10 illustrated in FIG. 1 will be described.

Initially, the coherent light source 21 of the projector 20 oscillatescoherent light under the control of the control device 35. The opticalpath of the light from the projector 20 is adjusted by thenon-illustrated scanning device, and the screen 40 is irradiated withthe light via the lenticular lens 70 and the Fresnel lens 80. Thescanning device adjusts the optical path so that the screen 40 isscanned with the light as illustrated in FIG. 2. The emission of thecoherent light from the coherent light source 21 is controlled by thecontrol device 35. The control device 35 stops the emission of thecoherent light from the coherent light source 21 according to the imageto be displayed on the screen 40. The operation of the scanning deviceincluded in the projector 20 is so fast that the operation is notresolvable by the human eye E. The observer thus simultaneously observesthe light projected on various positions on the screen 40, irradiated atseparate times.

The light projected on the screen 40 is transmitted through the firstcover layer 46 and the first electrode 41 to reach the optical sheet 50.This light is diffused and reflected by the particles 60 of the opticalsheet 50 and emitted in various directions on the observer side of thescreen 40. The observer can thus observe the reflected light from eachposition on the screen 40, at various positions on the observer side ofthe screen 40. Consequently, the observer can observe an imagecorresponding to the area irradiated with the coherent light on thescreen 40.

The coherent light source 21 may include a plurality of light sourcesthat emit coherent light of mutually different wavelength bands. In sucha case, the control device 35 controls the light source corresponding tolight of each wavelength band independently of the other light sources.As a result, a color image can be displayed on the screen 40.

In general, if an image is formed on a screen by using coherent light, adotted pattern of speckles is observed. One of the causes of thespeckles is considered to be that the coherent light, typified by laserlight, diffused over the screen produces an interference pattern on theoptical sensor surface or, in the case of human beings, on the retinas.In particular, if the screen is irradiated with coherent light by rasterscanning, the coherent light is incident on each position on the screenin a constant direction of incidence. If raster scanning is used, thespeckle wavefront occurring at each point of the screen remains fixedunless the screen swings. The speckle pattern, when observed with animage by the observer, significantly deteriorates the image quality ofthe display image.

By contrast, the screen 40 of the display apparatus 10 according to thepresent embodiment is configured to temporally change the diffusionwavefront. As the diffusion wavefront of the screen 40 changes, thespeckle pattern on the screen 40 changes temporally. If the temporalchange of the diffusion wavefront is made sufficiently high in speed,speckle patterns are superposed and averaged. This can make the specklesnot noticeable to the observer.

As illustrated in FIG. 1, the screen 40 includes the pair of first andsecond electrodes 41 and 42. The first electrode 41 and the secondelectrode 42 are electrically connected to the power source 30. In otherwords, the power source 30 can apply a voltage to the first electrode 41and the second electrode 42. If a voltage is applied between the firstelectrode 41 and the second electrode 42, an electric field is formed inthe optical sheet 50 lying between the first electrode 41 and the secondelectrode 42.

As illustrated in FIG. 4, the particles 60 including the first portion61 and the second portion 62 having different dielectric constants areoperably retained in the particle layer 55 of the optical sheet 50.Since the particles 60 are charged in the first place, or at least causedipole moment when an electric field is formed, the particles 60 operateaccording to the vector of the electric field formed. As the particles60 having a function of changing the traveling direction of light, suchas a reflection function and a diffusion function, operate to rotate inthe direction of the arrow A illustrated in FIG. 4, the diffusioncharacteristic of the screen 40 changes temporally. This can make thespeckles not noticeable to the observer.

The arrow La of FIG. 4 represents the image light projected onto thescreen 40 from the projector 20. The arrows Lb represent image lightdiffused by the screen 40. That the first portion 61 and the secondportion 62 of the particle 60 have different dielectric constants mayrefer to that the dielectric constants are different enough to exert aspeckle reducing function. Whether the first portion 61 and the secondportion 62 of the particle 60 have different dielectric constants canthus be determined based on whether the operably-retained particle 60can operate according to a change of the electric field vector.

The principle of operation of the particles 60 with respect to theretaining part 56 is that the particles 60 change in direction andposition so that the charges or the dipole moment of the particles 60comes to a positional relationship stable with respect to the electricfield vector. If a constant electric field is continuously applied tothe particle layer 55, the particles 60 therefore stop operating after acertain period of time. On the other hand, to make speckles notnoticeable, the operation of the particles 60 with respect to theretaining part 56 needs to be continued. The power source 30 thenapplies a voltage so that the electric field formed in the particlelayer 55 changes temporally. In the present embodiment, the power source30 applies a voltage between the first electrode 41 and the secondvoltage 42 so that the vector of the electric field generated in theoptical sheet 50 is reversed.

FIG. 9 illustrates an example of the voltage applied to the screen.

As illustrated in FIG. 9, voltages of X [V] and −Y [V] are alternatelyapplied between the first electrode 41 and the second electrode 42according to the present embodiment. The applied voltages of X [V] and−Y[V] may have the same or different absolute values. Voltages of threeor more different values may be applied. An applied voltage may changecontinuously, like when an ordinary alternating-current voltage isemployed.

The particles 60 are accommodated in the cavities 56 a formed in theretaining part 56. As illustrated in FIG. 4, the particles 60 have asubstantially spherical outer shape. The cavities 56 a to accommodatethe particles 60 have a substantially spherical inner shape. Theparticles 60 can thus rotate and vibrate about a rotation axis raextending in a direction perpendicular to the plane of FIG. 4. Note thatthe particles 60 can make not only repetitive rotational movements butalso translational movements depending on the size of the cavities 56 aaccommodating the particles 60. The cavities 56 a are filled with theliquid 57. The liquid 57 smoothens the operation of the particles 60with respect to the retaining part 56.

FIG. 10 illustrates a part of a cross section of the screen 40 accordingto another embodiment.

As illustrated in FIG. 10, the optical sheet 50 and the screen 40 may bemanufactured to a curved surface. To manufacture the optical sheet 50and the screen 40 to a curved surface, the particle layer 55 may beheated and polymerized on a curved surface during sheet formation. Then,the substrates 51 and 52 formed to the curved surface in advance may bestacked thereon.

FIG. 11 illustrates a particle 60 in an example where the first portion61 and the second portion 62 have different colors.

For example, in the foregoing embodiment, only either one of the firstand second portions 61 and 62 may include diffusion components. If onlyeither one of the first and second portions 61 and 62 includes diffusioncomponents, transmitted light is likely to be diffused by the diffusioncomponents even when the particle 60 changes in direction, orientation,or position. This can effectively make speckles not noticeable.

FIG. 12 illustrates a particle 60 in an example where the first portion61 and the second portion 62 have different volume ratios.

The foregoing embodiment has described an example where the firstportion 61 and the second portion 62 have the same volume ratios.However, such an example is not restrictive. The volume ratio of thefirst portion 61 in a particle 60 may be different from the volume ratioof the second portion 62 in the particle 60. In the example illustratedin FIG. 12, the volume ratio of the first portion 61 is higher than thatof the second portion 62. If such a particle 60 is used, the firstportion 61 can easily cover at least part of the second portion 62 fromthe observer side along a normal direction nd of the screen while thescreen 40 is irradiated with light. If the second portion 62 moves to aposition indicated by a two-dotted dashed line in FIG. 13 according tothe rotating operation of the particle 60, the first portion 61 canfully cover the second portion 62 from the observer side along thenormal direction nd of the screen 40. Even if the first portion 61 andthe second portion 62 do not have exactly the same color, a change inthe color tone of the screen 40 can thus be effectively made lessperceptible while an image is displayed with the particles 60 inoperation.

The foregoing embodiment has described an example where single-colorparticles 60 are fabricated from a monomer having positive chargeabilityand a monomer having negative chargeability by using synthetic resinpolymerization, and the particles 60 are charged. However, such anexample is not restrictive. Particles 60 including a plurality ofportions having different charging characteristics in the liquid 57 canbe synthesized by various methods using known materials. For example,particles 60 maybe fabricated by stacking two layers of plate-likemembers made of materials having different performances, and pulverizingthe stack into a desired size. Materials having a chargingcharacteristic may be formed, for example, by adding a charge controlagent to synthetic resin. Examples of charging additives include an ionconductivity imparting agent used as an antistatic agent. The ionconductivity imparting agent can be formed by compounding lithiumperchlorate or the like with a polymer chiefly containing polyalkyleneglycol.

The foregoing embodiment has described an example where the particles 60are spheres. However, such an example is not restrictive. The particles60 may have spheroidal, cubic, rectangular solid, conical, cylindrical,and other outer shapes as long as the particles 60 can operate insidethe cavities 56 a. The operation of the particles 60 having anon-spherical outer shape can temporally change the diffusioncharacteristic of the screen 40 by surface reflection, without aninternal diffusion function of the particles 60.

The optical sheet 50, the particle layer 55, and the particles 60 may bemanufactured by a method different from that described in the foregoingembodiment. The liquid 57 does not need to be provided if the particles60 are operably retained with respect to the retaining part 56.

The foregoing embodiment has described an example of the layeredstructure of the screen 40. However, this is not restrictive. The screen40 may include other functional layers that are expected to providecertain functions. One functional layer may provide two or morefunctions. For example, the first cover layer 46, the second cover layer47, the first substrate 51, the second substrate 52, or the like mayserve as the functional layer. Examples of the functions to be given toa functional layer may include an anti-reflection function, ahard-coating function with abrasion resistance, an ultraviolet shieldingfunction, an ultraviolet reflection function, and an anti-stainfunction.

FIG. 13 illustrates an example of a screen 40 including a plurality oflinear electrode portions.

The foregoing embodiment has described an example where the firstelectrode 41 and the second electrode 42 are formed flat and arranged tosandwich the particle layer 55. However, such an example is notrestrictive. At least either one of the first and second electrodes 41and 42 may be formed in stripes. In the example illustrated in FIG. 13,both the first electrode 41 and the second electrode 42 are formed intransparent stripes. More specifically, the first electrode 41 includesa plurality of first linear electrode portions 41 a extending linearly.The plurality of first linear electrode portions 41 a is arranged in adirection orthogonal to their longitudinal direction. Like the firstelectrode 41, the second electrode 42 also includes a plurality ofsecond linear electrode portions 42 a extending linearly. The pluralityof second linear electrode portions 42 a is arranged in the directionorthogonal to their longitudinal direction.

In the example illustrated in FIG. 13, the plurality of first linearelectrode portions 41 a constituting the first electrode 41 and theplurality of second linear electrode portions 42 a constituting thesecond electrode 42 are both arranged on a surface on the same side ofthe optical sheet 50 as the observer is. The plurality of first linearelectrode portions 41 a constituting the first electrode 41 and theplurality of second linear electrode portions 42 a constituting thesecond electrode 42 are alternately arranged along the same arrangementdirection. The first electrode 41 and the second electrode 42illustrated in FIG. 13 can also form an electric field in the particlelayer 55 of the optical sheet 50 by application of a voltage from thepower source 30.

The foregoing embodiment has described an example where the projector 20projects light on the screen 40 by the raster scan method. However, suchan example is not restrictive. The projector may use a method other thanthe raster scan method. For example, the projector may project imagelight on the entire area of the screen at each instant. Speckles canoccur even if such a projector is used. However, the use of theforegoing screen can temporally change the diffusion wavefront of thescreen 40 and effectively make the speckles not noticeable. Moreover,the foregoing screen can be used in combination with the projectordisclosed in International Publication 2012/033174, described in theBackground Art section. According to such a projector, speckles can beeffectively reduced. The combination of the projector and the foregoingscreen can further effectively make speckles not noticeable.

FIG. 14 illustrates apart of a cross section of a transmission typescreen 140 according to another embodiment.

The transmission type screen 140 includes a diffuse transmission layer58 added to the optical sheet 50 of the screen 40 illustrated in FIG. 3.The diffuse transmission layer 58 has a function of diffusing lightduring transmission. The rest of the configuration of the transmissiontype screen 140 may be the same as that of the screen 40 illustrated inFIG. 3.

The diffuse transmission layer 58 of the transmission type screen 140may be provided between the second substrate 52 and the second electrode42. Non-illustrated diffusion particulates 58 a may be embedded in thesecond substrate 52 so that the second substrate 52 functions as thediffuse transmission layer 58. The non-illustrated diffusionparticulates 58 b may be included in the retaining part 56 or the liquid57 of the particle layer 55 so that the particle layer 55 also functionsas the diffuse transmission layer 58.

FIG. 15 illustrates an example of a display apparatus 10 using thetransmission type screen 140 according to the present embodiment.

The transmission type display apparatus 10 according to the presentembodiment includes a projector 20 and the screen 140 which isirradiated with image light from the projector 20. Here, the screen 140may include at least the electrodes 41 and 42, the particle layer 55,and the diffuse transmission layer 58 among the components illustratedin FIG. 14.

The display apparatus 10 may use at least one of a Fresnel lens 70arranged between the projector 20 and the screen 140, a viewing angleexpansion part 71 for expanding the viewing angle of the screen 140, anda coloring part 72 for improving contrast. To improve the durability ofthe screen 140, a hard coat part 73 may be used.

The Fresnel lens 70 refracts the light emitted from the projector 20 andemits the light as substantially parallel light. The Fresnel lens 70 isarranged between the projector 20 and the screen 140. The Fresnel lens70 may be either a concentric Fresnel lens or a linear Fresnel lenshaving a similar effect to that of a cylindrical lens. A Fresnel surface70 a of the Fresnel lens 70 may be directed toward either the projector20 or the screen 140. The Fresnel lens 70 may be formed in a sheet-likeshape.

Such a Fresnel lens 70 can be used to emit the light emitted from theprojector 20 as substantially parallel light, whereby image quality onthe screen 140 can be improved. The screen 140 can be formed thinly byforming the Fresnel lens 70 in a sheet shape.

The viewing angle expansion part 71 expands the viewing angle of thescreen 140. In a first example illustrated in FIG. 15, the viewing angleexpansion part 71 includes a substrate 71 a of transparent resin, air,or the like, and light absorption portions 71 b formed like blacktriangular prisms in the substrate 71 a. The substrate 71 a has arefractive index higher than that of the light absorption portions 71 b.

The light emitted from the projector 20 is incident on the substrate 71a side of the viewing angle expansion part 71 and totally reflected bythe slopes at the interfaces between the substrate 71 a and the lightabsorption portions 71 b to expand the viewing angle. Little lighttherefore enters the light absorption portions 71 b, whereby the useefficiency of the light can be increased. The light absorption portions71 b absorb external light from the observer E side. This can improvecontrast.

While the viewing angle expansion part 71 illustrated in FIG. 15 isformed by arranging the light absorption portions 71 b in the verticaldirection of the diagram to vertically diffuse light, the viewing angleexpansion part 71 may be formed by arranging the light absorptionportions 71 b in a direction from the near side to the far side of thediagram to laterally diffuse light. The viewing angle expansion part 71including the light absorption portions 71 b arranged in the verticaldirection of the diagram and the viewing angle expansion part 71including the light absorption portions 71 b arranged in the directionfrom the near side to the far side of the diagram may be stacked andused between the projector 20 and the observer E. The use of the stackedviewing angle expansion parts 71 can efficiently implement a screenhaving a wide viewing angle and high contrast without lowering the useefficiency of the light.

The coloring part 72 is a semitransparent layer or a layer colored withcoloring dye, etc. The coloring part 72 lowers transmittance to improvecontrast. The coloring part 72 does not need to be provided as anindependent layer, and other parts may have a similar function. Forexample, the retaining part 56, the liquid 57, and the particles 60 maybe colored for improved contrast. If a laser is used as the lightsource, the coloring part 72 may be colored to transmit only brightlines.

The hard coat layer 73 is formed by coating the outermost layer of thescreen with a known hard coating material, and improves durability. Thehard coat layer 73 may have water permeability or water repellency. Forexample, if the screen 140 is used in a low temperature environment, dewcan condense on the hard coat layer 73. The surface of the hard coatlayer 73 may therefore be formed by fluorine, silicon, or other coating,an uneven structure, attachment of a moisture absorbing layer, anelectric heating layer, or the like. Dew condensation and stains can beprevent by thus forming the surface of the hard coat layer 73.

The display apparatus 10 may be equipped with a touch panel function.According to an infrared optical imaging method, an infrared sensor isprovided in part of the display apparatus 10. If the display apparatus10 is irradiated with infrared rays from outside, the infrared sensordetects the position of a touch on the display apparatus 10 as alocation where the infrared irradiation is blocked, whereby a touchpanel function is provided. The infrared irradiation portion may be aseparately-provided irradiation device. Similarly, according to anultrasonic method, a transmitter and a receiver of ultrasonic surfaceelastic waves are installed on the surface of the display apparatus 10.A contact position is identified from attenuation of the elastic waves.A capacitive or resistive touch panel may be provided inside orexternally attached to the display apparatus 10. If the touch panel isprovided inside the display apparatus, the touch panel layer may beformed between the screen 140 and the coloring part 72 illustrated inFIG. 16, or between the coloring part 72 and the hard coat layer 73.

FIG. 16 illustrates another example of the Fresnel lens 70.

A surface of the Fresnel lens 70 opposite from the Fresnel surface 70 amay be formed as a diffusion surface 70 b having diffusibility. Thediffusion surface 70 b can be formed by making the surface uneven. Theformation of such a diffusion surface 70 b can improve the diffusibilityof the entire screen.

FIG. 17 illustrates a second example of the viewing angle expansionpart.

A viewing angle expansion part 171 according to the second exampleillustrated in FIG. 17 includes a lenticular lens 171 a serving as asubstrate, and light absorption portions 171 b formed on part of thelenticular lens 171 a.

The lenticular lens 171 a includes minute slender first cylindricallenses 171 a ₁ arranged on the projector 20-side surface of the sheet,and minute slender second cylindrical lenses 171 a ₂ and flat surfaceportions 171 a ₃ alternately formed on the observer E-side surface ofthe sheet. The flat surface portions 171 a ₃ of the lenticular lens 171a form the light absorption portions 171 b. The lenticular lens 171 a ismade of transparent resin. The light absorption portions 171 b areformed by black coating or the like.

The light emitted from the projector 20 is incident on the firstcylindrical lenses 171 a ₁ of the lenticular lens 171 a of the field ofviewing angle expansion part 171, and emitted from the secondcylindrical lenses 171 a ₂ to expand the viewing angle. Little light istherefore incident on the light absorption portions 171 b, whereby theuse efficiency of light can be improved. The light absorption portions171 b absorb external light from the observer E side. This can improvecontrast.

While the viewing angle expansion part 171 illustrated in FIG. 17 isformed by arranging the first cylindrical lenses 171 a ₁ and the secondcylindrical lenses 171 a ₂ in the vertical direction of the diagram tovertically diffuse light, the viewing angle expansion part 171 may beformed by arranging the first cylindrical lenses 171 a ₁ and the secondcylindrical lenses 171 a ₂ in the direction from the near side to thefar side of the diagram to laterally diffuse light. The field of viewingangle expansion part 171 including the first cylindrical lenses 171 a ₁and the second cylindrical lenses 171 a ₂ arranged in the verticaldirection of the diagram and the field of viewing angle expansion part171 including the first cylindrical lenses 171 a ₁ and the secondcylindrical lenses 171 a ₂ arranged in the direction from the near sideto the far side of the drawing may be stacked and used between theprojector 20 and the observer E. The use of the stacked field of viewingangle expansion parts 171 can efficiently implement a screen having awide viewing angle and high contrast without lowering the use efficiencyof the light.

The viewing angle expansion part 171 may be a microlens array or thelike. The light absorption portions 171 b may be a pinhole array or thelike.

FIG. 18 illustrates a third example of the viewing angle expansion part.

A viewing angle expansion part 271 according to the third exampleillustrated in FIG. 18 includes a lenticular lens 271 a serving as asubstrate, and a light absorption portion 271 b formed on part of thelenticular lens 271 a.

The lenticular lens 271 a includes minute slender cylindrical lenses 271a ₁ arranged on the projector 20-side surface of the sheet. A flatsurface portion 271 a ₂ is formed on the observer E-side surface of thesheet. The lenticular lens 271 a is formed of transparent resin. Thelight absorption portion 271 b is formed to cover the cylindrical lenses271 a ₁ of the lenticular lens 271 a. The light absorption portion 271 bis formed in layers by coating or the like.

The light emitted from the projector 20 is incident on the lightabsorption portion 271 b of the field of view expansion part 271, andemitted from the flat surface portion 271 a ₂ to expand the viewingangle. The light absorption portion 271 b repeats internal reflection toattenuate external light from the observer E side. This can improvecontrast.

While the viewing angle expansion part 271 illustrated in FIG. 18 isformed by arranging the cylindrical lenses 271 a ₁ in the verticaldirection of the drawing to vertically diffuse light, the viewing angleexpansion part 271 may be formed by arranging the cylindrical lenses 271a ₁ in the direction from the near side to the far side of the drawingto laterally diffuse light. The v field of viewing angle expiation part271 including the cylindrical lenses 271 a ₁ arranged in the verticaldirection of the drawing and the field of viewing angle expansion part271 including the cylindrical lenses 271 a ₁ arranged in the directionfrom the near side to the far side of the drawing may be stacked andused between the projector 20 and the observer E. The use of the stackedfield of viewing angle expansion parts 271 can efficiently implement ascreen having a wide viewing angle and high contrast without loweringthe use efficiency of the light.

The viewing angle expansion part 271 maybe a microlens array or thelike.

Next, the diffuse transmission layer 58 according to the presentembodiment will be further described. In the present embodiment, thediffuse transmission layer 58 may include the diffusion particulates 58a to provide the function of diffusing light.

In the case of the diffuse transmission layer 58, the diffusionparticulates 58 a may be made of materials such as resin beads andsilicon dioxide which have a small refractive index difference from themain component such as acrylic resin. The diffusion particulates 58 athat have a small refractive index difference from the main componentare likely to cause forward scattering and are preferably used fortransmission type.

The diffuse transmission layer 58 may be separated from the particlelayer 55. There may be a layer of air between the diffuse transmissionlayer 58 and the particle layer 55. The diffuse transmission layer 58may be provided on both sides of the particle layer 55.

FIG. 19 illustrates another example of the diffuse transmission layer58.

The diffuse transmission layer 58 may be configured as a diffusetransmission layer 58 by forming irregular protrusions and recesses onthe surface. In such a case, the protrusions and recesses of the diffusetransmission layer 58 may be formed on the particle layer 55 side. Asillustrated in FIG. 19, the diffuse transmission layer 58 may beseparated from the particle layer 55. There may be a layer of airbetween the diffuse transmission layer 58 and the particle layer 55. Thediffuse transmission layer 58 may be provided on both sides of theparticle layer 55.

Next, the particles 60 will be further described. In the presentembodiment, as illustrated in FIG. 4, the particles 60 are multiphasepolymer particles, and include the first portion 61 and the secondportion 62. The first portion 61 and the second portion 62 may bereferred to as a first polymer portion 61 and a second polymer portion62.

In the case of a reflection type particle 60, the first main portion 66a and the second main portion 67 a of the particle 60 may be made ofacrylic resin or the like. The first diffusion components 66 b and thesecond diffusion components 67 b may be made of a metal compound such astitanium oxide which has a large refractive index difference from thefirst main portion 66 a and the second main portion 67 a. The first andsecond main portions 66 a and 67 a and the first and second diffusioncomponents 66 b and 67 b having a large refractive index differencetherebetween are likely to cause backward scattering and are preferablyused for reflection type.

In the case of a transmission type particle 60, the first main portion66 a and the second main portion 67 a of the particle 60 may be made ofacrylic resin or the like. The first diffusion components 66 b and thesecond diffusion components 67 b may be made of material such as silicondioxide which has a small refractive index difference from the firstmain portion 66 a and the second main portion 67 a. The first and secondmain portions 66 a and 67 a and the first and second diffusioncomponents 66 b and 67 b having a small refractive index differencetherebetween are likely to cause forward scattering and are preferablyused for transmission type.

In the particle layer 55, the retaining part 56, the liquid 57, and theparticles 60 may be colored with a coloring agent.

FIGS. 20A and 20B illustrate the concept of a cavity 56 a to a particle60.

In the present embodiment, a single particle 60 is included in a singlecavity 56 a. A single cavity 56 a refers to a unit beyond which aparticle 60 is not movable. In the example illustrated in FIG. 20A, twocavities 56 a include respective particles 60. In the exampleillustrated in FIG. 20B, three cavities 56 a include respectiveparticles 60. In other words, a single particle 60 is included in asingle cavity 56 a.

In the example of the particle 60 illustrated in FIG. 4, the firstdiffusion components 66 b and the second diffusion components 67 b arespheres. However, such an example is not restrictive. The particles 60may have a spheroidal, cubic, rectangular solid, conical, cylindrical,or other outer shapes.

If the first diffusion components 66 b and the second diffusioncomponents 67 b have a shape such as a spheroidal, cubic, rectangularsolid, conical, or cylindrical shape, the diameter refers to an areaequivalent circular diameter (Heywood diameter). An area equivalentcircular diameter d can be determined from the following equation:

d=(4×S/π)^(1/2),

where

-   d is the area equivalent circular diameter, and-   S is an area obtained from a transmission electron microscope (TEM)    image of the cross section of the diffusion component.

The particles 60 preferably have a diameter of 30 μm to 200 μm. If theparticles 60 are not spherical in shape, an area equivalent circulardiameter d is also determined from the foregoing equation.

The diameter is obtained from a photomicrograph or a scanning electronmicroscope (SEM) image. A volume fraction is determined from an areafraction obtained from a SEM image or TEM image of the cross section ofthe particle. If the area fraction is x:y, the volume fraction isx^(3/2):y^(3/2).

To determine a refractive index difference, slice samples of theparticle are initially fabricated by using a focused ion beam (FIB).Qualitative/quantitative elemental analyses of the main portions and thediffusion components are performed by using an apparatus in which atransmission electron microscope or scanning electron microscope iscombined with a detector of energy dispersive X-ray spectrometry (EDX)or electron energy-loss spectroscopy (EELS). A chemical state analysisof the elements is further performed if needed. The components of themain portions and the diffusion components are estimated, and adifference between the refractive indexes of the respective componentsis determined.

In the present embodiment, transparent refers to that when a slicefabricated by a microtome is measured for transmittance and reflectanceby microspectroscopy, the transmittance exceeds the reflectance at anywavelength in the visible light region.

As described above, according to an example of the optical sheet 50 ofthe present embodiment, there is provided the particle layer 55including the transparent retaining part 56 that has a predeterminedthickness and the particles 60 that are accommodated in the cavities 56a formed in the retaining part 56 and include the first portion 61 andthe second portion 62 having different dielectric constants. The firstportion 61 includes the transparent first main portion 66 a and thefirst diffusion components 66 b that diffuse light. The second portion62 includes the transparent second main portion 67 a and the seconddiffusion components 67 b that diffuse light. The first diffusioncomponents 66 b and the second diffusion components 67 b have a diameterd satisfying the conditional expression (1) below. This can reducedifferences in the scattering characteristics of the respective R, G,and B colors of the laser projector, and reduce color unevenness atlocations such as the center and the four corners of the screen 40 ordue to the viewing angle. Laminar flows of two compositions are formedin the microchannel, so that particles 60 can be smoothly formed andclogging of the flow channel or the inlet of the flow channel can besuppressed.

0.1 μm<d<15 μm   (1)

According to an example of the optical sheet 50 of the presentembodiment, the volume fraction Vi indicating the proportion of the sumof the volumes of the first diffusion components 66 b and the seconddiffusion components 67 b to the volume of the particle 60 satisfies theconditional expression (2) below. Beams not contributing to specklereduction can thus be reduced as much as possible.

Vi≥3%   (2)

According to an example of the optical sheet 50 of the presentembodiment, a refractive index difference Δn between the first andsecond diffusion components 66 b and 67 b and the first and second mainportions 66 a and 67 b satisfies the conditional expression (3) below.This causes forward scattering more than backward scattering, andincreases transmitted beams.

|Δn|≥0.2   (3)

According to an example of the screen 40 of the present embodiment,there are provided the optical sheet 50 and the electrodes 41 and 42which form an electric field for driving the particles 60 in theparticle layer 56 when a voltage is applied thereto. The particles 60including the first portion 61 and the second portion 62 havingdifferent dielectric constants can thus be precisely operated accordingto the electric field formed by the electrodes 41 and 42. The operationof the particles 60 having a reflection diffusion function cantemporally change the diffusion characteristic of the screen 40 forsufficient reduction of speckles.

According to an example of the screen 40 of the present embodiment, theelectrodes 41 and 42 have a function of diffusing and reflecting lightas the diffuse reflection layer 53. A new diffuse reflection layer 53therefore does not need to be provided, and the optical sheet 50 can bereduced in thickness.

According to an example of the display apparatus 10 of the presentembodiment, there are provided the screen 40 and the projector 20 whichirradiates the screen 40 with coherent light. Speckles of the coherentlight projected from the projector 20 can thus be sufficiently reduced.

According to an example of the display apparatus 10 of the presentembodiment, there are further provided the power source 30 which appliesa voltage to the electrodes 41 and 42 of the screen 40, and the controldevice 35 which controls the applied voltage applied from the powersource 30 to the electrodes 41 and 42. The control device 35 controlsthe applied voltage from the power source 30 so that the particles 60operate in the particle layer 55. The particles 60 can thus be operatedmore precisely, and speckles can further be sufficiently reduced.

According to an example of the display apparatus 10 of the presentembodiment, the control device 35 controls the applied voltage from thepower source 30 to repeatedly rotate the particles 60 within an angularrange of less than 180°. At least either one of the first and secondportions 61 and 62 can thus be selected and precisely positioned on theobserver side.

According to an example of the display apparatus 10 of the presentembodiment, the control device 35 controls at least either one of thedirection and the position of the particles 60 by the applied voltage ofthe power source 30 so that the first portion 61 covers at least part ofthe second portion 62 from the observer side along the normal directionof the screen 40. Even if the first portion 61 and the second portion 62do not have exactly the same color, a change in the color tone of thescreen 40 can thus be effectively made less perceptible while an imageis displayed with the particles 60 in operation.

While the optical sheet 50, the screen 40, and the display apparatus 10have been described based on several embodiments, the present inventionis not limited to such embodiments, and various combinations ormodifications may be made.

Explanation of Reference Symbols

10: display

20: projector

21: coherent light source

30: power source

35: control device

40: screen

41: first electrode

42: second electrode

46: first cover layer

47: second cover layer

50: optical sheet

51: first substrate

52: second substrate

55: particle layer

56: retaining part

56 a: cavity

57: liquid

60: particle

61: first portion

66 a: first main portion

66 b: first diffusion component

62: second portion

67 a: second main portion

67 b: second diffusion component

1. An optical sheet comprising a particle layer including a transparentretaining part that has a predetermined thickness, and a particle thatis accommodated in a cavity formed in the retaining part and includes afirst portion and a second portion having different dielectricconstants, wherein the first portion includes a transparent first mainportion and a first diffusion component that diffuses light, the secondportion includes a transparent second main portion and a seconddiffusion component that diffuses light, and the first diffusioncomponent and the second diffusion component have a diameter dsatisfying the following conditional expression (1):0.1 μm<d<15 μm   (1)
 2. The optical sheet according to claim 1, whereina volume fraction Vi indicating a proportion of a sum of volumes of thefirst diffusion component and the second diffusion component to a volumeof the particle satisfies the following conditional expression (2):Vi≥3%   (2)
 3. The optical sheet according to claim 2, wherein arefractive index difference Δn between the first and second diffusioncomponents and the first and second main portions satisfies thefollowing conditional expression (3):|Δn|<0.2   (3)
 4. A screen comprising: the optical sheet according toclaim 1; and an electrode that forms an electric field for driving theparticle of the particle layer when a voltage is applied thereto.
 5. Thescreen according to claim 4, wherein the electrode has a function ofdiffusing and reflecting light as a diffuse reflection layer.
 6. Adisplay apparatus comprising: the screen according to claim 4; and aprojector that irradiates the screen with coherent light.
 7. The displayapparatus according to claim 6, further comprising: a power source thatapplies a voltage to the electrode of the screen; and a control devicethat controls an applied voltage from the power source to the electrode,wherein the control device controls the applied voltage from the powersource to operate the particle in the particle layer.
 8. The displayapparatus according to claim 7, wherein the control device controls theapplied voltage from the power source to repeatedly rotate the particlewithin an angular range of less than 180°.
 9. The display apparatusaccording to claim 7, wherein the control device controls at leasteither one of a direction and a position of the particle by the appliedvoltage from the power source so that the first portion covers at leastpart of the second portion from an observer side along a normaldirection of the screen.
 10. A screen comprising: the optical sheetaccording to claim 2; and an electrode that forms an electric field fordriving the particle of the particle layer when a voltage is appliedthereto.
 11. A screen comprising: the optical sheet according to claim3; and an electrode that forms an electric field for driving theparticle of the particle layer when a voltage is applied thereto.
 12. Adisplay apparatus comprising: the screen according to claim 5; and aprojector that irradiates the screen with coherent light.
 13. Thedisplay apparatus according to claim 8, wherein the control devicecontrols at least either one of a direction and a position of theparticle by the applied voltage from the power source so that the firstportion covers at least part of the second portion from an observer sidealong a normal direction of the screen.